42 COMPARATIVE ELECTRO-PHYSIOLOGY 



responses from the line of absolute equilibrium, they will be 

 found to increase with increasing stimulus. Ordinarily, how- 

 ever, no allowance is made for the shifting of the base-line, 

 the responses being measured instead from the place of its 

 previous recovery, or point of modified equilibrium. In this 

 way these responses undergo an apparent diminution. 



I have occasionally observed another curious phenomenon 

 in connection with the subject of response under increasing 

 stimulus. During the gradual increase of the stimulus from 

 a low value, there would at first be no response. But on 

 reaching a certain critical value, a response would suddenly 

 be evoked which was maximum that is to say, would not be 

 exceeded, even when the stimulus was further increased. 

 We have here a parallel case to what is known in animal 

 physiology as the 'all or none' principle. In the case of 

 cardiac muscle, for example, there is a certain minimal 

 intensity of stimulus which is effective in inducing response. 

 But further increase of stimulation causes no concomitant 

 increase of effect. 



When a tissue is subjected to rapidly succeeding stimuli, 

 the excitatory effects are superposed upon each other. In 

 muscle, for example, the contractile effect of the second 

 stimulus is added to that of the first, before that has time to 

 disappear. The result is a summation of effects more or less 

 complete ; and these attain a maximum. With moderate 

 frequency of stimulation, such a tetanic effect is incomplete, 

 tending to become more and more complete, with the 

 progressive increase of frequency (fig. 52). I have obtained 

 results in every way similar to these, with the mechanical 

 response of ordinary plants. In fig. 33 is given a photo- 

 graphic record of tetanus, taken from the longitudinal motile 

 responses of the style of Datura alba. Similar tetanic effects 

 are also obtained in the electric response of plants, of which 

 the records seen in fig. 34 form an example. 



The difficulties in the quantitative observation of electrical 

 response have thus been overcome by the employment of two 

 different methods of stimulation namely, torsional vibration, 

 and stimulation by thermal shocks. In the case of the 



